A critical step to ultra-high-speed all-optical data transmission

June 10, 2015

A polarized light field microscope image shows crystal junctions written inside glass with a femtosecond laser. Upon divergence (a), independent lattice orientations develop in each branch and are retained (b) as the branches merge back into a single line. The color wheel indicates the angle of the fast or slow axis of birefringence (credit: Lehigh University)

Researchers from Lehigh University, Japan, and Canada have advanced a step closer to the dream of all-optical data transmission by building and demonstrating what they call the “world’s first fully functioning single-crystal waveguide in glass.”

In an open-access article published in Scientific Reports, a Nature publication, the group said it had employed ultrafast femtosecond lasers to produce a three-dimensional single crystal capable of guiding light waves through glass with little loss of light.

The group says its achievement will boost ongoing efforts to develop photonic integrated circuits (PICs) that are smaller, cheaper, more energy-efficient and more reliable than current networks that use bulky discrete optoelectronic components — waveguides, splitters, modulators, filters, amplifiers —- to transport optical signals.

“A major trend in optics,” the researchers write, “has been a drive toward … replacing systems of large discrete components that provide individual functions with compact and multifunctional PICs, in much the same way that integration of electronics has driven the impressive advances of modern computer systems.”

A need for 3D fabrication techniques

To make this transition, however, improved methods of fabricating 3D PICs are needed, the researchers say.

“The methods currently employed for fabricating PICs are photolithographic [used in fabricating chips] and other processes suitable for planar [flat] geometries,” the researchers write. “3D PIC fabrication techniques would enable a much higher density of components and much more compact devices, while at the same time creating opportunities for new technologies such as high density 3D optical memory.”

To fabricate 3D PICs, say the researchers, it is necessary, first, to prevent light from scattering as it is being transmitted and, second, to transmit and manipulate light signals fast enough to handle increasingly large quantities of data.

Glass, an amorphous material with an inherently disordered atomic structure, cannot meet these challenges, the researchers say. Crystals, with their highly ordered specific lattice structure, have the requisite optical qualities.

Femtosecond lasers + ferroelectric materials

To pattern crystals in glass, the Lehigh-led group employed femtosecond lasers, whose speed and precision make them useful for cataract and other eye surgeries. A femtosecond is one-quadrillionth, or 10-15 of a second. Pulses emitted by femtosecond lasers last between a few femtoseconds and hundreds of femtoseconds.

Scientists have been attempting for years to make crystals in glass to prevent light from being scattered as light signals are transmitted, says Jain. The task is complicated by the “mutually exclusive” nature of the properties of crystal and glass.

Glass turns to crystal when it is heated, says Jain, but it is critical to control the transition. “The question is, how long will this process take and will we get one crystal or many. We want a single crystal; light cannot travel through multiple crystals. And we need the crystal to be in the right shape and form.”

After conducting experiments at Lehigh and at Kyoto University and Polytechnique Montreal, the group built a single crystal in glass, demonstrated its waveguiding capabilities and quantified its transmission efficiency. The glass and crystal both were composed of lanthanum borogermanate (LaBGeO5), a ferroelectric material.

“We achieved quality,” says Dierolf, “by guiding light from one end of the crystal to the other with very little loss of light. “We have made the equivalent of a wire to guide the light. With our crystal, it is possible to do this in 3D so that the wire and the light can curve and bend as it is transmitted. This gives us the potential of putting different components on different layers of glass.”

The fact that the demonstration was achieved using ferroelectric materials is another plus, says Dierolf. “Ferroelectric crystals have demonstrated an electrical-optical effect that can be exploited for switching and for steering light from one place to another as a supermarket scanner does. Ferroelectric crystals can also transform light from one frequency to another. This makes it possible to send light through different channels.”

“Other groups have made crystal in glass but were not able to demonstrate quality,” says Jain. “With the quality of our crystal, we have crossed the threshold for the idea to be useful. As a result, we are now exploring the development of novel devices for optical communication in collaboration with a major company.”

The femtosecond laser provides several critical advantages, say Dierolf and Jain. The high intensity of the laser pulse enables nonlinear optical absorption. The precise focus enables researchers to control where the laser is focused and where light is absorbed. The unique focus of the femtosecond laser also makes it possible to “write” the crystal inside the glass and not on its surface.

Direct three-dimensional laser writing of amorphous waveguides inside glass has been studied intensely as an attractive route for fabricating photonic integrated circuits. However, achieving essential nonlinear-optic functionality in such devices will also require the ability to create high-quality single-crystal waveguides. Femtosecond laser irradiation is capable of crystallizing glass in 3D, but producing optical-quality single-crystal structures suitable for waveguiding poses unique challenges that are unprecedented in the field of crystal growth. In this work, we use a high angular-resolution electron diffraction method to obtain the first conclusive confirmation that uniform single crystals can be grown inside glass by femtosecond laser writing under optimized conditions. We confirm waveguiding capability and present the first quantitative measurement of power transmission through a laser-written crystal-in-glass waveguide, yielding loss of 2.64 dB/cm at 1530 nm. We demonstrate uniformity of the crystal cross-section down the length of the waveguide and quantify its birefringence. Finally, as a proof-of-concept for patterning more complex device geometries, we demonstrate the use of dynamic phase modulation to grow symmetric crystal junctions with single-pass writing.